Joshua J. Caron

The Bleustein–Gulyaev wave for liquid sensing applications

Abstract Crystallographic orientations in piezoelectric materials which support the Bleustein–Gulyaev (BG) wave are shown to offer sensor designers candidate sensor platforms for liquid phase sensing applications. This wave, which occurs in the family of Z-axis cylinder cuts in piezoelectric materials belonging to the orthorhombic mm2 crystal group, has displacements in the surface plane perpendicular to the propagation direction and an electric field in the sagittal plane. This wave offers the advantage of not radiating a compressional wave when loaded with a liquid. Theoretical calculations for the velocity and the associated mechanical and electric fields have been performed for potassium niobate (KNbO3), a candidate mm2 group crystal, which show that this material offers many promising orientations for liquid phase acoustic wave sensors.

A surface acoustic wave mercury vapor sensor

A sensor for the in situ detection and measurement of low concentrations of gaseous mercury is presented. The sensor is based upon a dual delay line SAW oscillator with a gold-coated delay path. Gaseous mercury interacts rigorously with the gold film, forming an amalgam. The resulting increase in film mass is manifested as a decrease in oscillation frequency. Measurement of gas concentration is achieved by differentiating the sensor response at room temperature or by operating the sensing element at a temperature where gas-film reaction kinetics result in equilibrium rates of mercury amalgamation and desorption. This equilibrium value of amalgamated mercury is highly dependent upon the gas concentration. Thus, the delay line oscillation frequency is a sensitive measure of gaseous mercury concentration. Responses of this sensor to gaseous mercury concentrations in the ppb range are presented. The sensor response features are analyzed in terms of response shape, response magnitude, response time, and linearity at 25/spl deg/C and 200/spl deg/C.

The integration of a chemiresistive film overlay with a surface acoustic wave microsensor

Abstract The theory describing the operation of a surface acoustic wave (SAW) hydrogen sulfide (H 2 S) sensor was examined. This sensor employs a tungsten trioxide (WO 3 ) chemiresistive overlay deposited as the sensing element on a SAW delay line. Previous work has shown that a change in the films DC conductivity upon exposure to a target gas, such as H 2 S, has far more influence on the SAW than the films mechanical properties. However, early work also pointed out that higher sensitivities had been experienced experimentally than the current theory suggests. The objective of the current work was to reexamine the theory describing the effect of a films electrical properties on SAW velocity. Perturbation theory has been employed to characterize the effects of the films DC conductivity, dielectric constant and device operating frequency on the SAW gas sensor. Experimental work was conducted on a SAW delay line residing on a cut of quartz optimized for stability at high temperature.

Theory, design and operation of a surface acoustic wave hydrogen sulfide microsensor

Abstract The theory, design and operation of a surface acoustic wave (SAW) hydrogen sulfide (H 2 S) sensor that uses a gold-doped semiconducting metal oxide, tungsten trioxide (WO 3 :Au), film as the gas sensing element is described. It is shown experimentally that the dominant property change that occurs in the film when exposed to H 2 S is the electrical conductivity. Finally, it is pointed out that upon exposure to a target gas, a properly designed SAW gas sensor that monitors large changes in film electrical conductivity is much more sensitive than a similar sensor that monitors mechanical properties such as mass loading.

A surface acoustic wave nitric oxide sensor

A dual delay line surface acoustic wave (SAW) device employing a ruthenium-doped tungsten trioxide (WO/sub 3/:Ru) film as a sensing element for nitric oxide (NO) is presented. Exposure of this semiconducting metal oxide (SMO) film to NO results in oxidation of the film, decreasing the films carrier concentration and, therefore, its conductivity. This decrease in film conductivity causes a corresponding increase in the SAW velocity. Therefore, when the device is configured as a dual delay line oscillator, the frequency of the sensing channel becomes a sensitive measure of NO concentration. Responses of this sensor to NO concentrations in ppb to low ppm levels in air along with higher concentrations of possible interferent gases are presented. The sensor response features are analyzed in terms of response time, recovery time, minimum detection limit, saturation detection limit and linearity. Conclusions are drawn and future improvements to the sensor are suggested.

Surface acoustic wave substrates for high temperature applications

A systematic search of several piezoelectric substrates and crystallographic orientations has been conducted in an effort to identify temperature compensated cuts. As a result of this search, a sequence of crystallographic orientations along the rotated Y-cut (RYC) in quartz has been identified as appropriate for operation over a broad range of temperatures. Theoretical calculations, in fact, show that different orientations along RYC quartz provide compensation at temperatures ranging from -35/spl deg/C to over 500/spl deg/C, thereby allowing one to select an appropriate temperature stable orientation for a particular application. Experimental measurements performed on selected RYCs in quartz have confirmed these predictions of temperature stability.

Effects of film thickness on sensitivity of SAW mercury sensors

An ST cut quartz 262 MHz dual delay line surface acoustic wave (SAW) mercury vapor sensor has been designed, fabricated, and rested. A gold sensing film is deposited on one delay path while the second delay path remains bare and acts as a reference to cancel out frequency amalgamations due to variations in temperature, gas flow rate, etc. The sensors ability to detect mercury is due to the strong interaction between gold and mercury, known as amalgamation. In the present work a large number of gold films of various thicknesses have been exposed to mercury concentrations from 20 to 100 parts per billion (ppb) in dry nitrogen. It is shown that the steady-stale response magnitude is highest for a 75 Å gold film with decreasing sensitivity for films which are thinner of thicker. A model is presented which explains this behavior.

Electrical parasitic modeling in SAW RF filters

Parasitic phenomena affecting the frequency response of SAW RF filters are modeled with a variety of techniques. Inter-IDT electrode capacitance is modeled with a two-dimensional charge distribution analysis. Capacitance among arbitrary die structures, including bus bars and bond pads, is modeled with an electrostatic boundary element formulation. Inductance and resistance of these structures are modeled using a filament mesh analysis technique under the magnetoquasistatic approximation.

The Bleustein-Gulyaev wave mode in potassium niobate for liquid sensing applications

The Surface acoustic wave (SAW) velocities and the associated mechanical and electric fields for the Z-axis cylinder family of cuts in potassium niobate (KNbO/sub 3/) have been theoretically calculated. It is found that these cuts support pure shear horizontal (SH) SAWs or Bleustein-Gulyaev (BG) waves with displacements in the surface plane and perpendicular to the propagation direction and an electric field in the sagittal plane. The piezoelectric coupling coefficient is predicted to have a maximum of 53%, which is significantly larger than any SAW coupling coefficient observed in LiNbO/sub 3/, LiTaO/sub 3/ or quartz. Since the SH SAW is not damped when loaded with a liquid, the Z-axis cylinder cuts of KNbO/sub 3/ may offer promising piezoelectric substrates for liquid phase acoustic wave sensors.

Temperature stable piezoelectric substrates for SAW gas sensors

Abstract A systematic search of several surface acoustic wave (SAW) substrates and crystallographic orientations has been conducted in an effort to identify temperature compensated cuts. As a result of this search, a sequence of crystallographic orientations along the rotated Y-cut (RYC) in quartz has been identified as appropriate for operation over a broad range of temperatures. Theoretical calculations, in fact, show that different orientations along RYC quartz provide compensation at temperatures ranging from −35°C to or over 500°C, thereby allowing one to select an appropriate temperature stable orientation for a particular application. Experimental measurements performed on selected RYCs in quartz have confirmed these predictions of temperature stability.